1. Field of the Invention
This invention relates to coating compositions and more particularly, to aqueous resin dispersions which are useful as baking paints for metals, particularly for interior surfaces of cans, and are able to form excellent films adapted for these purposes.
2. Description of the Prior Art
Conventional can coating paints and corrosion-resistant paints have been heretofore desired to be switched over to aqueous systems from the standpoint of resource and energy savings, and environmental pollution. In aqueous paints, epoxy resins have been chiefly investigated as in the case of solvent-type paints. In fact, a number of methods of dispersing epoxy resins in water were proposed.
For instance, surface active agents are used to disperse epoxy resins in water. For this purpose, two methods are known including a method using anionic surface active agents and a method using nonionic surface active agents. In the former method, oxysilane rings open during emulsification and storage with a lowering of reactivity. This leads to deterioration of the film-forming property. In some case, thickening and gelling problems may take place. In the latter method, fairly large amounts of surface active agents are contained in the system in order to improve dispersability and storage stability, so that the surface active agents tend to give adverse influences on chemical and mechanical properties of a formed film.
To solve the above problems, there have been proposed a variety of self-emulsifiable epoxy resins in which epoxy resins are modified with acrylic resins to introduce emulsifiable segments into the epoxy resin molecules.
For example, Japanese Laid-open Patent Application No. 53-1228 describes a grafted epoxy resin which is obtained by polymerizing a monomer mixture containing a carboxylic acid monomer in an epoxy resin by the use of a free radical generator such as benzoyl peroxide. This grafted epoxy resins is stably dispersed in an aqueous medium containing bases.
Japanese Laid-open Patent Application No. 56-43362 described a water-bearing coating composition which comprises a reaction product of an epoxy resin, a carboxyl group-containing polymer and a tertiary amine. Moreover, Japanese Laid-open Patent Application Nos. 53-14963, 55-9433 and 58-40363 disclose that partial reaction products, in excess of carboxyl groups, which are obtained by reaction between acrylic resins and relatively high molecular weight aromatic epoxy resins are stably dispersed in aqueous medium in the presence of ammonia or amines. In addition, Japanese Laid-open Patent Application Nos. 55-3481 and 55-3482 describe self-emulsifiable epoxy ester copolymers, i.e. carboxyl group-bearing function polymers are esterified with epoxy resins in the presence of amine esterification catalysts to obtain resins which are substantially free of any oxirane group, and the thus obtained resins are self-emulsified in water with the aid of bases.
Japanese Laid-open Patent Application Nos. 57-105418 and 58-198513 disclose aqueous dispersion compositions which are obtained by polymerizing low molecular weight compounds, which are produced by partial reaction between aromatic epoxy resins and (meth)acrylic acid and which have epoxy groups and acryroyl groups in one molecule thereof, and a monomer mixture comprising acrylic acid or methacrylic acid, and neutralizing the resulting copolymer with basic compounds.
The self-emulsifiable epoxy resins obtained by the above process has no surface active agent in the paints, strong films are obtained from the paints. If a higher curing speed is needed, water-soluble amino resins or phenolic resins may be added to these paints. When used in an appropriate amount, water-soluble amino resins serve to improve the curing speed without lowering physical properties of the film. In some case, because the crosslinking density increases, a suitable degree of film hardness can be obtained. However, when the paint comprising a water-soluble amino resin is applied for interior surface coating of cans, a serious problem arises in that low molecular weight compounds derived from the amino resin are dissolved out in the content of a can by thermal sterilization treatment. Addition of phenolic resins to aqueous dispersion compositions of the reaction products of aromatic epoxy resins and acrylic resins are described, for example, in the afore-indicated Japanese Laid-open Patent Application Nos. 55-3481, 55-3482, 56-43362 and 58-3482. However, the dispersion compositions comprising ordinary phenolic resins are still unsatisfactory in rapid curability.
Japanese Laid-open Patent Application No. 57-182361 teaches a paint which consists of a resol-type phenolic resin, which essentially contains a phenol dimer component, an alcoholic hydroxyl group-bearing material, and organic solvents.